The present invention is directed to a cover layer for micro-electronic components and VLSI circuits. In particular, the present invention is directed to a cover layer having a high absorbency for alpha radiation.
As VLSI circuits are becoming increasingly miniaturized, not only does the geometry decrease, but, the electrical values such as, for example, the capacitance of a memory cell, constructed as a capacitor, are reduced to a minimum value. The circuits thereby become increasingly susceptible to external disturbances such as, for example, radioactive emission. In particular, highly ionizing alpha radiation can erase or cause the falsification of data stored as charges in circuits containing memory cells.
To protect the circuits against alpha radiation "from the inside", the circuits are constructed from high purity initial materials that exhibit only a minimum amount of radioactive elements or, respectively, isotope contaminants. To protect the circuits against "external radiation", the circuits have anti-radiation protective layers. These layers typically consist of photo-cross-linkable polyimide lacquers. A layer thickness of about 30 micrometers is sufficient to provide reliable protection of the circuit against alpha rays.
Typical anti-radiation layers, however, exhibit various disadvantages. In a method for applying these layers, the "raw", uncross-linked polyimide resin is applied surface-wide on a wafer containing the circuit. The resin layer is then cross-linked with ultraviolet light in the regions over the circuits, but only over the latter chips. Subsequently these regions are developed. After stripping the unexposed regions, the remaining polyimide layer is after-cured in a multi-hour temperature treatment up to about 250.degree. C. The polyimide layers shrink during this tempering process, resulting in a layer that is only up to 50% that of the original volume. This shrinkage causes the circuit lying below these layers to become mechanically stressed and can thereby damage or even destroy the circuit.